Thin channel-on-insulator MOSFET device with n+ epitaxy substrate and embedded stressor

ABSTRACT

A method of forming a field effect transistor (FET) device includes forming a recess in a PFET region of a starting semiconductor substrate comprising a bulk semiconductor layer an epitaxial n+ layer formed on the bulk semiconductor layer, a buried insulator (BOX) layer formed on the epitaxial n+ layer, and an active semiconductor or silicon-on-insulator (SOI) layer formed on the BOX layer, the recess being formed completely through the SOI layer, the BOX layer, and partially into the epitaxial n+ layer; epitaxially growing a silicon germanium (SiGe) transition layer on the epitaxial n+ layer, the SiGe transition layer having a lower dopant concentration than the epitaxial n+ layer; and epitaxially growing embedded source/drain (S/D) regions on the SiGe transition layer and adjacent the SOI layer in the PFET region, the embedded S/D regions comprising p-type doped SiGe.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.14/221,490, filed Mar. 21, 2014, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates generally to semiconductor devicemanufacturing techniques and, more particularly, to a thinchannel-on-insulator metal oxide semiconductor field effect transistor(MOSFET) device having an n+ epitaxy substrate with an embeddedstressor.

In the manufacture of integrated circuit devices it is desirable toreduce the dimensions of the transistors used to form the individualcircuits. In the case of field effect transistors (FETs), reducing thechannel length provides the capability to deliver a given amount ofdrive current with a smaller channel width. By reducing the width andlength of a FET, the parasitic gate capacitance (which is a function ofthe area defined by the width and length) can be reduced, therebyimproving circuit performance. Similarly, reducing the size oftransistors is beneficial in that less area is consumed for a givencircuit, and this allows more circuits in a given area, or a smaller,less costly chip, or both.

To further enhance transistor performance, MOSFET devices have beenfabricated using strained channel regions located in portions of asemiconductor substrate. Strained channel regions allow enhanced carriermobility to be realized, thereby resulting in increased performance whenused for n-channel (NFET) or for p-channel (PFET) devices. Generally, itis desirable to induce a tensile strain in the n-channel of an NFET inthe source-to-drain direction to increase electron mobility and toinduce a compressive strain in the p-channel of a PFET in thesource-to-drain direction to increase hole mobility. Currently, thereare several existing approaches of introducing strain in the transistorchannel region.

In one approach, strain in the channel is introduced by creating arecess in the substrate in the source/drain regions. For example, a PFETdevice having a compressive stress in the channel region may be formedon a silicon substrate by epitaxially growing a stress-inducing layerhaving a larger lattice structure than the silicon, such as a layer ofSiGe, within recessed regions in the source/drain regions. Similarly, anNFET device having a tensile stress in the channel region may be formedon a silicon substrate by epitaxially growing a stress-inducing layerhaving a smaller lattice structure than the silicon, such as a layer ofSiC, within recessed regions in the source/drain regions.

SUMMARY

In an exemplary embodiment, a method of forming a field effecttransistor (FET) device includes forming a recess in a PFET region of astarting semiconductor substrate comprising a bulk semiconductor layeran epitaxial n+ layer formed on the bulk semiconductor layer, a buriedinsulator (BOX) layer formed on the epitaxial n+ layer, and an activesemiconductor or silicon-on-insulator (SOI) layer formed on the BOXlayer, the recess being formed completely through the SOI layer, the BOXlayer, and partially into the epitaxial n+ layer; epitaxially growing asilicon germanium (SiGe) transition layer on the epitaxial n+ layer, theSiGe transition layer having a lower dopant concentration than theepitaxial n+ layer; and epitaxially growing embedded source/drain (S/D)regions on the SiGe transition layer and adjacent the SOI layer in thePFET region, the embedded S/D regions comprising p-type doped SiGe.

In another embodiment, a method of forming a field effect transistor(FET) device includes defining an a PFET region and an NFET region of astarting semiconductor substrate comprising a bulk semiconductor layeran epitaxial n+ layer formed on the bulk semiconductor layer, a buriedinsulator (BOX) layer formed on the epitaxial n+ layer, and an activesemiconductor or silicon-on-insulator (SOI) layer formed on the BOXlayer; forming gate stack structures on the SOI layer in the NFET andPFET regions; forming a recess in the PFET region, the recess beingformed completely through the SOI layer, the BOX layer, and partiallyinto the epitaxial n+ layer; epitaxially growing a silicon germanium(SiGe) transition layer on the epitaxial n+ layer, the SiGe transitionlayer having a lower dopant concentration than the epitaxial n+ layer;epitaxially growing embedded source/drain (S/D) regions on the SiGetransition layer and adjacent the SOI layer in the PFET region, theembedded S/D regions comprising p-type doped SiGe; and forming raisedS/D regions on portions of the SOI layer in an NFET region of thestarting semiconductor substrate.

In another embodiment, a field effect transistor (FET) device includes asemiconductor substrate comprising a bulk semiconductor layer anepitaxial n+ layer formed on the bulk semiconductor layer, a buriedinsulator (BOX) layer formed on the epitaxial n+ layer, and an activesemiconductor or silicon-on-insulator (SOI) layer formed on the BOXlayer; an epitaxial silicon germanium (SiGe) transition layer grown onthe epitaxial n+ layer in a PFET region of the semiconductor substrate,the SiGe transition layer having a lower dopant concentration than theepitaxial n+ layer; and embedded source/drain (S/D) regions epitaxiallygrown on the SiGe transition layer and adjacent the SOI layer in thePFET region, the embedded S/D regions comprising p-type doped SiGe.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1 through 7 are a series of cross sectional views illustrating amethod of forming a thin channel-on-insulator MOSFET device having an n+epitaxy substrate with an embedded stressor, in which:

FIG. 1 illustrates a starting substrate including a bulk layer, an n+epitaxial layer, a BOX layer and an active layer;

FIG. 2 illustrates the formation of exemplary NFET and PFET activeregions in the active layer of FIG. 1;

FIG. 3 illustrates the formation of patterned transistor gate structuresover the NFET and PFET regions of FIG. 2;

FIG. 4 illustrates recessing portions of the PFET active region, throughthe BOX layer and into the n+ epitaxial layer;

FIG. 5 illustrates the formation of epitaxially grown, lightly dopedn-type SiGe regions on exposed portions of the n+ epitaxial layer;

FIG. 6 illustrates the formation of epitaxially grown, doped p+ SiGeregions on exposed portions of the n-type SiGe regions; and

FIG. 7 illustrates the formation of raised source/drain regions on theNFET active region.

DETAILED DESCRIPTION

With respect to the channel strain engineering techniques discussedabove, for a thin channel MOSFET such as an extremely thinsilicon-on-insulator ETSOI substrate or a FinFET where the channel isseparated from the substrate with an insulator, it is desirable to forma recess into the dielectric that separates the active layer from thebulk substrate and thereafter grow embedded S/D regions by formingepitaxy using the substrate as the starting plane. In the case ofembedded dynamic random access memory (eDRAM) devices, it is desirableto use substrates with n+ epitaxy layer directly beneath the buriedoxide (insulator) (BOX) layer. However, forming embedded S/D regionsdirectly on an n+ substrate is not previously known.

Accordingly, disclosed herein is a method of forming embedded S/Dregions on n+ substrates for PFET devices. In order to avoid extrajunction leakage due to the relatively high doping level of the n+substrate (e.g., 1×10¹⁹ atoms/cm³ or higher) and the relatively highdoping level of the p+ embedded S/D regions (e.g., 1×10²⁰ atoms/cm³ orhigher), a lesser doped n-type layer is initially formed between the n+substrate and the p+ S/D regions.

Referring generally now to FIGS. 1 through 7, there are shown a seriesof cross sectional views illustrating a thin channel-on-insulator MOSFETdevice having an n+ epitaxy substrate with an embedded stressor, inaccordance with an exemplary embodiment.

As shown in FIG. 1, a starting semiconductor substrate 100 includes abulk semiconductor layer or handle portion 102 having an epitaxiallygrown, n+ layer 104 formed thereon. As indicated above, the epitaxial n+layer 104 may be of particular used in fast, high density eDRAM deviceapplications, and may be formed to an exemplary thickness of about 7 toabout 10 microns (μm). As also indicated above, the epitaxial n+ layer104 may have an n-type dopant concentration on the order of about 1×10¹⁹atoms/cm³ or higher. A buried insulator (e.g., oxide) (BOX) layer 106 isformed on the epitaxial n+ layer 104, to an exemplary thickness of about10 to about 100 nanometers (nm), and an active semiconductor orsilicon-on-insulator (SOI) layer 108 is formed on the BOX layer 104. Theactive SOI layer 108 may be formed to an exemplary thickness of about 4to about 10 nm, and includes a semiconductor material from which FETdevice channels are to be defined.

The active SOI layer 108 is patterned so as to define active NFET andPFET regions 110, 112, respectively, as shown in FIG. 2. In the exampleillustrated, the active NFET region 110 and active PFET region 112 areformed by mesa isolation. However, it should be appreciated that otherforms of active device isolation may be employed such as shallow trenchisolation (STI) formation, for example. Then, as shown in FIG. 3, gatestructures 114 are formed on the active NFET and PFET regions 110, 112.The gate structures 114 may include, for example, one or more gateinsulating layers 116 (e.g., oxide, oxynitride, high-k material, etc.),one or more gate conductor layers 118 (e.g., polysilicon, metal, metalnitride, etc.), and one or more sets of sidewall spacers 120 (e.g.,nitride). It will be appreciated that the figures are not depicted toscale and that multiple such gate structures 114 may be formed on theactive NFET and PFET regions 110, 112. Although not specificallydepicted in FIG. 3, halo and extension regions may optionally be formedas known in the art (such as by ion implantation, for example) adjacentto and/or beneath the gate structures 114.

Referring now to FIG. 4, a mask layer 122 is formed over the device toprotect the active NFET region 110 and is lithographically patterned soas to expose the active PFET region 112. The mask layer 122 may includeone or more of a photoresist layer and a hardmask layer, for example.Recesses 124 are then formed (such as by etching) through the exposedportions of the active PFET region 112, corresponding to PFET source anddrain (S/D) locations. The recesses 124 further extend completelythrough the BOX layer 106 and into the epitaxial n+ layer 104, and mayhave a total depth from about 25 to about 75 nm.

As then shown in FIG. 5, an n-type SiGe transition layer 126 isepitaxially grown on the epitaxial n+ layer 104. The n-type SiGetransition layer 126 is formed so as to have a lower dopantconcentration than that of the epitaxial n+ layer 104. In an exemplaryembodiment, the concentration of the n-type dopant of the transitionlayer 126 may be on the order of about 1×10¹⁸ to about 3×10¹⁸ atoms/cm³,roughly an order of magnitude lower than that of the epitaxial n+ layer104. The doping for the n-type SiGe transition layer 126 may beaccomplished by ion implantation or, alternatively, by in situ doping,and the thickness of the n-type SiGe transition layer 126 may be on theorder of about 10 to about 50 nm. More specifically, the thickness ofthe n-type SiGe transition layer 126 may be on the order of about 20 toabout 40 nm.

Proceeding to FIG. 6, a p++ SiGe layer 128 is epitaxially grown on then-type SiGe transition layer 126. The p++ SiGe layer 128 serves asembedded S/D regions for a PFET device, and provides a compressivestress on a channel region defined in the active PFET region 112. Anexemplary thickness of the p++ SiGe layer 128 is on the order of about30 to about 80 nm, and has a p-type dopant concentration on the order ofabout 1×10²⁰ atoms/cm³ or higher. Previously, such embedded S/D regionshave not been grown for PFET devices used in eDRAM devices having an n+layer below the BOX layer. The presence of the lightly doped n-type SiGetransition layer 126 provides a gradual transition from the more heavilydoped n+ layer 104 to the heavily doped p++ SiGe layer 128, whichotherwise might result in excessive junction leakage current through aheavily doped p-n junction.

Following processing of the PFET region 112, FIG. 7 illustratesprocessing for the NFET region 110. Another mask layer 130 is formedover the device and patterned in a manner so as to protect the PFETregion 112 and expose the NFET region 110 for further processing. In theembodiment depicted, raised S/D regions 132 are epitaxially grown on thesemiconductor surfaces of the NFET region 110. The raised S/D regions132 may be n++ doped regions of epitaxially grown silicon (Si) forexample, or alternatively, silicon carbon (Si:C), and have a p-typedopant concentration on the order of about 1×10²⁰ atoms/cm³ or higher.

Although the illustrated process flow is depicted as having firstprocessed the PFET region 112 to form the embedded S/D regions, followedby processing the NFET region 110 to form the raised S/D regions, itshould be appreciated that a different order of processing operations isalso contemplated. That is, for example, the epitaxial growth of theraised S/D regions 132 on the semiconductor surfaces of the NFET region110 as shown in FIG. 7 could be performed prior to etching the PFETregion 112 and growing the lightly doped n-type SiGe transition layer126 and heavily doped p++ SiGe layer 128 as shown in FIGS. 4-6.

While the invention has been described with reference to an exemplaryembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

The invention claimed is:
 1. A field effect transistor (FET) device,comprising: a semiconductor substrate comprising a bulk semiconductorlayer, an epitaxial n+ layer formed on and in direct contact with thebulk semiconductor layer, a buried insulator (BOX) layer formed on theepitaxial n+ layer, and an active semiconductor or silicon-on-insulator(SOI) layer formed on the BOX layer; an epitaxial silicon germanium(SiGe) transition layer arranged in direct contact with the BOX layerand in direct contact with the epitaxial n+ layer in a PFET region ofthe semiconductor substrate, the SiGe transition layer having a lowerdopant concentration than the epitaxial n+ layer; and embeddedsource/drain (S/D) regions epitaxially grown on the SiGe transitionlayer and adjacent the SOI layer in the PFET region, the embedded S/Dregions comprising p-type doped SiGe.
 2. The device of claim 1, whereinthe SiGe transition layer is doped at a concentration of about 1×10¹⁸atoms/cm³ or higher.
 3. The device of claim 1, wherein the n+ epitaxiallayer is doped at a concentration of about 1×10¹⁹ atoms/cm³ or higher.4. The device of claim 1, wherein the embedded S/D regions are doped ata concentration of about 1×10²⁰ atoms/cm³ or higher.
 5. The device ofclaim 1, further comprising a gate stack structure on the SOI layer inthe PFET region.
 6. The device of claim 1, wherein the epitaxial n+layer has a thickness of about 7 to about 10 microns (μm).
 7. The deviceof claim 1, wherein the SiGe transition layer has a thickness of about10 to about 50 nanometers (nm).
 8. The device of claim 1, wherein theembedded S/D regions have a thickness of about 30 to about 80 nm.
 9. Thedevice of claim 1, further comprising raised S/D regions grown onportions of the SOI layer in an NFET region of the semiconductorsubstrate.